Adaptive optical beam alignment and link protection switching for 5G-over-FSO.

Free-space optics (FSO) convey an enormous potential for ultra-high-capacity seamless fiber-wireless transmission in 5G and beyond communication systems. However, for its practical exploitation in future deployments, FSO still requires the development of very high-precision and robust optical beam alignment. In this paper, we propose two different methods to achieve tight, precise alignment between a pair of FSO transceivers, using a gimbal-based setup. For scenarios where there is no information about the system, a black-box artificial intelligence (AI)-based method resorting to particle swarm optimization (PSO) is presented, enabling to autonomously align the system with a success rate above 96%, converging from a blind starting position. Alternatively, for scenarios with partial information about the FSO system, we propose a tailored custom algorithm, with a success rate of 92%, but with a ∼4 × reduction on the alignment time. The automatic alignment is then validated in a 5G-like fiber-FSO scenario, transmitting a 16 × 400 MHz signal and achieving a maximum bit-rate of 30 Gbps. Moreover, we propose the implementation of a fail-safe mechanism with a backup FSO receiver, thereby providing an extra degree of robustness towards temporary events of strong degradation on the FSO channel or line-of-sight (LOS) interruption.